Photoelectric surface



Aug. 30,1938. B. c. GARDNER' 2,128,582

PHOTOELECTRICfSURFACE Filed Aug. 18, 1955 Pfg/:1.

ExHA usr I I .ALkAL/ METAL, Fw.

Cwwn wie A TTRNEYS.

Patented Aug. 30,1938

PATENT oFFicE morosmic'rmc simmer:

'Benina c. Gardner, Philadelphie, Pe., assigner to Farnsworth Television Incorporated, San I Francisco, Calif., a corporation of California i Appiieetien Angustia, 193s, serial No. 96,613

'5 claims.

My invention relates to photoelectric tubes and, more particularly, to a means-and method for obtaining a photoelectric surface of high sensitivity,

'andmy method in its preferred form is suitable for' the production of photosurfaces highly sensi- -tive in the red end of the. spectrum.

Among the objects of my invention are: To provide ameans .and method-.oi obtaining a photosensitive surface'of high eiiiciency; to pro- A vide a means and method of obtaining photosurfaces highly sensitive in the red end of the specgether with the foregoing, will be set forth in the following description of specic apparatus embodying and utilizing my n ovel method. It is. therefore to be understood that my method is apl plicable to other apparatus, and kthat I vdo not limit myself, in any way, to the apparatus of the present application, as I may adopt variousother apparatus embodiments, utilizing the method, within the scope of the appended claims.

In the drawing:

Figure' 1 is a diagram showing a tube'and apparatus setup by which the photosurface .of my invention may be formed.

Figure 2 is a cross-sectional view of the tube shown in Figure 1.

As .my invention relates inv its preferred. form scribe the forming'of my surface in conjunction with what is known as a caesium-silver oxide tube.

Heretofore, in making such tubes a silver'surface is oxidized and caesium admitted thereilifterg" meanwhile checking `the photoelectric emission from the tube, and the admission of caesium is 'stopped as soon as the photoelectric emission reaches a predetermined and desired value. -At

this point there is considerable free caesium left within the container, and this' free caesiuin is highlydetrimental to the final operation ofthe device, inasmuch as leakage currents vdevelop due to the fact that caesium metal is a good conductor-of electricity even inl extremely thinillms.

It is customary, therefore, after the tube has reeiieatiie desired sensitivity, te retake the een-l tainer until thefree alkali metal isremoved from the tube. The alkali metalA which has been deposited on the silver oxide is retained therein and is not re-evaporated during this baking process, and the sensitivity` of the formed surface does not suffer appreciably by the baking.

.It is, however, extremely important in the prior art devices that no excess caesium be admitted beyond that needed to properly sensitize the surface ,inasmuch as. the output sensitivity will be lowered and cannot be restored under the ordi-l nary procedure. The prior artprocesses, therefore, are all characterized by ,requiring constant attention and carefulsupervision, and in spite of such supervision .tubes vary greatly in their sensitivity.

My method allows the use of excess quantities 'of alkali metal, the sensitization being accomplished' by oxidization of the final deposit. In addition, I do not remove the excess alkali metal from the tube, but simply change it from a metal to an oxide, the latter being non-conductive, `thus leakage is eliminated.

The more detailed advantages of my new method may be more fully understood by a direct reference to the formation of a caesium-silver oxide surface in accordance with the method. Referring to the drawing:

An envelope i is provided with a cathode foundation member 2 and an anode 3. The tube is sealed onto an exhaust line 4 to whichoxygen may be supplied through a stopcock or other suitable vmeans 5. Caesium metal is placed in a still tube t by which the metal may be Vessel. ,f 'f

In the .normal formation/of a caesium-silver oxide surface I prefer to use for a cathode foundation pure electrolitic, silver form.. Before being mounted in the envelope I prefer to clean the eventual'actlve surface with a mild abrasive, suchv s that known in the trade as fBon Ami", and thoroughly'wash the plate. The silver plate is then etched by passing therepassed intothe r'olled into sheet "over a hydrogen flame until by eye the entire then pieeed' within the vessel l and the tube isI evacuated and'baked. After the electrodes have been thoroughly cleaned of adsorbed gases, oxygen iii-admitted to thetube under low pressure and a glowdischarge is struck between anode 3 and cathode 2. This glow discharge may be either alternating currentlor direct current. The etchedv surface is then watched as it passes' through a number of color changes, and I have found that the most eilicient', oxide layer is formed when the surface passes through yellow, brown, blue white, back to yellow, and. then into a second brown. When the second brown color is obtained the tube may be pumped out and rebaked at 200 for ten minutes. The caesium metal is then distilled in through the still tube 6 in excess quantity, and no attempt need be made to put in any denite quantity, though I usually prefer to admit about twice the amount which would normally be needed. The silver turns a bright golden color due to the deposition on the oxide layer of a relatively large amount of metallic caesium. Un-

der normal circumstances this relatively large.

amount of metallic caesium would run or now on the cathode plate, but I have found that the etching step described above prevents such iiow and consequent irregularities. The etching insures that the caesium will be deposited uniformly upon the silver oxide. At this stage the cathode is sensitive only to the point that metallic caesium would give a photoelectric, response and in addition, the walls of the tube have enough metallic caesium thereon to cause sufficient leakage to prevent proper operation of the device.

I then readmit oxygen'. and the caesiumsurface changes color and appears almost black. Oxygen again is admitted until all conductivity along the Walls of the device is killed. If no convenient connection can be made within the tube to determine Whether or not conductivity across a link is destroyed, then I may prefer to insert a pair of spaced contacts l and d and join them by a circuit concerning a battery 9 and an indieating device ill. necessary in specific cases where contacts for determining leakage cannot readily be obtained with the normal electrode structure.

Normally all leakage conductivity due to metallic caesium is destroyed before the photo-current is developed to its maximum,-and consequently, after conductivity has been destroyed, additional oxygen is slowly added, meanwhile checking the photo-current in an indicating device il. 4It is found that the photoelectric current will increase up to thepoint when thermionic emission starts.

vIt is easy, of course, to define where this point is,

by intermittently exciting a. light source adjacent the tube. If the current in the photo-current device indicator i i, joining the anode and cathode through photo anode battery l2, persists after the light is turned out, then the thermionic emission point has been reached, and I have foundthat the peak of the .photoelectric current is reached when the thermionic current lis between zero andl one hundred microamperes from a surface area of apprommately nine' square inches at 200 centigrade.

The process as above described produces photoelectric surfaces having a considerably higher sensitivity than the usual caesium-silver oxide surfaces, and the surfaces produced have a remarkable uniformity. Furthermore, I have found that the process is not critical and may be repeated in case a slip in procedure takes place. For

example, if oxygen is admitted in the final step in' too great a quantity,-so that the photo-current passes its peak and starts to drop, the sensitivity of the surface may be restored by admitting a little more caesium and re-oxidizing as described for However, this latter is onlyv the original caesium deposit. Thus I have been able to produce surfaces having not only a high sensitivity, but which are reproducible in quantity. Steps which are hard to control, such as the admission of caesium, are no longer critical, and the steps which are easy to control, such as the admission of oxygen, are Ysuch that they may readily be checked as to the action thereof.

The high sensitivity of the surface, and in addition, the uniformity of surface with respect to adjacent areas of the cathode, together with complete lack of leakage due tothe non-conductivity of caesium oxide, make the surface I have just describedideally adapted for use in televisiondissector tubes utilized for electronic scanning. It should also be pointed out that the process is ideally adapted for use inconjunction withmosaic surfaces; For example, if the basic silver foundation is in the form of a silver mosaic, and this mosaic be oxidized and caesium admitted in accordance with my process herein described, then it is obvious that when the final oxygen treatment is administered the metallic caesium between the silver islands is oxidized, and that therefore the islands will be electrically distinct. One of the main problems of forming a photoelectric mosaic under present methods is the elimination of caeslum coating between islands. It should also be pointed out that while I have described my process as used in conjunction with a silver foundation which is self-oxidized, and having a particular alkali metal,l namely, caesium, deposited on the oxide, it is obvious that my method may be applied to other equivalent foundation materials upon which caesium will deposit, and that alkali metals, other than caesium, can be used for the photosensitive material.

I claim:

l. The method of forming photoelectric surfaces in an envelope comprising forming therein a `substance with which alkali metal will combine,

admitting an excess of alkali metal, admitting oxygen until all uncombined metal is oxidized, and continuing the admission of oxygen until thermionic emission is obtained from said surface at 200 -centigrada 2. The method of forming photoelectrlc surfaces in an envelope comprising depositing a silver surface therein, oxidizing said silver surface admitting an excess of alkali metal, admitting oxygen until all metal other than that directly assoelated withl said silver oxide is changed to alkali metal oxide, and continuing the admission of oxygen until thermionic emission appears from said surface at 200 centigrade.

3. A photoelectric surface comprising in order a silver layer, a' silver oxide layer, a caesium metal layer, and a caesium oxide layer.

e. A method of preparing a silver surface to receive a deposit of photoelectric material which comprises the step of etching said surface in at hydrogen flame.

5. A-method of preparing a silver surfaceV to- 

